雾化喷涂沉积对晶界扩散Nd-Fe-B磁性能和微观结构的影响

Influence of Spray Coating Deposition on Magnetic Properties and Microstructure of Grain Boundary Diffusion Nd-Fe-B Magnets

摘要:

提出了一种新的雾化喷涂沉积（SCD）方法，在Nd-Fe-B磁体表面均匀沉积TbF₃粉末，同时通过晶界扩散过程（GBDP）将Tb元素引入到磁体中。用这种方法（SCD+GBDP）处理厚度达5 mm的钕铁硼磁体。研究了TbF₃涂层增重比、扩散时间和扩散温度对烧结磁体组织和磁性能的影响。样品扩散温度和时间为940 ℃和10 h，退火温度和时间为480 ℃和5 h。TbF₃增重比（w）从0%增加到0.8%时，磁体的矫顽力从1201 kA/m 提高到1930 kA/m，剩磁下降约0.01 T。研究发现，随着TbF₃增重比的增加，磁体的矫顽力先增大后减小。SEM结果表明，在Nd₂Fe₁₄B晶粒边界区域，Tb取代Nd形成(Nd, Tb)₂Fe₁₄B核壳相。晶界相和核壳相中较高的磁晶各向异性对矫顽力的增强有积极的促进作用。核壳相的分布和浓度对矫顽力有密切的影响。当TbF₃增重比大于2.4%时，靠近磁体表面区域的晶界扩散明显增强。元素的SEM图像显示，进入磁体的Tb越多，晶核内的Tb浓度就越高。此外，大量Nd-F/Nd-O-F相的形成导致晶界相不像w=0.8% 时的样品那样连续，这可能是导致矫顽力下降的主要原因。

Abstract:

A novel method of spray coating deposition (SCD) was proposed to deposit TbF₃ powder solution uniformly onto the surface of Nd-Fe-B magnets, and then the Tb element was introduced into the magnets through the grain boundary diffusion process (GBDP). Nd-Fe-B magnets with thickness up to 5 mm were treated with this method (SCD+GBDP). The effects of mass gain ratio (w) of TbF₃ coatings, diffusion time and the diffusion temperature on the microstructure and magnetic properties of the sintered magnets were investigated. The samples were diffused at 940 °C for 10 h and then annealed at 480 °C for 5 h. Results show that when the TbF₃ mass gain ratio increases from 0% to 0.8%, the coercivity of the magnets increases from 1201 to 1930 kA/m and the remanence is only decreased by 0.01 T. With increasing the mass gain ratio of TbF₃, the coercivity of the magnets increases firstly and then decreases obviously. SEM results show that the (Nd, Tb)₂Fe₁₄B core-shell phases can be formed by Tb which replaces the Nd in the boundary region among Nd₂Fe₁₄B grains. The improved decoupling effect by the continuous grain boundary phase and the higher magnetocrystalline anisotropy of the core-shell phase plays a positive role in the coercivity enhancement. The distribution and concentration of core-shell phase have a close influence on the coercivity. When the TbF₃ mass gain ratio is more than 2.4%, the grain boundary diffusion is obviously enhanced in the region close to the magnet surface. The SEM image of the element shows that the more Tb enters the inside of the grain, the higher the Tb concentration in the core than in the shell. Furthermore, the formation of a large amount of Nd-F/Nd-O-F phase causes the grain boundary phase not being as continuous as that of the sample with w=0.8%, which may be the main reason for the decrease in coercivity.